Method of determining and adjusting the aerodynamic pitching moment of a full-sized aerodynamic member and apparatus therefor



Sept. 6, 1955 G. s. DOMAN 2,716,889

METHOD OF DETERMINING AND ADJUSTING THE AERODYNAMIC PITCHING MOMENT OF A FULL-SIZED AERODYNAMIC MEMBER AND APPARATUS THEREFOR Filed Nov. 25, 1952 INVENTOR. filz'adzw 61 Jamm @AZLfMMyW 17 TTURA/EYS United States Patent Office 2,716,889 Patented Sept. 6, V 1955 METHOD OF DETERMINING AND ADJUSTING THE AERODYNAMIC PITCHING MOMENT OF A FULL-SIZED AERODYNAMIC MEMBER AND APPARATUS THEREFOR Gliddeu S. Doman, Trumbull, Conn., assiguor to Doman Helicopters, Inc., Danbury, Conn., a corporation of Delaware Application November 25, 1952, Serial No. 322,508 18 Claims. (Cl. 73-147) The invention relates to a method of testing or determming the aerodynamic pitching moment or its coefficient of a short span of a considerably longer full-sized aerodynamic member and particularly of a blade of a selfsustaining aircraft such as a helicopter blade. The invention also relates to the determining of the pitching moment or the moment coefiicient and the adjustment of the aerodynamic member or blade to change the same to a desired value. The invention further relates to an apparatus that no two blades are satisfactorily identical in their operational behavior, that is no two blades will track when in operation on a rotor head. It is for this reason that adjusting means is provided on a helicopter blade. This adjusting means takes the form of a bendable trailing edge tab on each blade and extending the length thereof, which is bent either up or down to correct the aerodynamic pitching moment of the blade at points along its entire length so that the blades of a set of blades when mounted upon a rotor head will track each other. A set of helicopter rotor blades are presently tested (for tracking by mounting them upon a whirling stand at p the factory and turning the rotor at full operating speed and determining whether or not the blades are tracking.

The head is stopped, the blades are adjusted and then whirled again which process is continued until the blades do satisfactorily track each other. This is a time consuming process, requires high horsepower and elaborate measuring equipment. Since blades are matched as a .set, when it is necessary to change one blade, either all blades must be changed in order to install a matched set or the newly inserted blade must be whirled and adjusted '-until it tracks the other blades on the head.

The invention subjects a'relatively small span of a I full-sized aerodynamic member such as a helicopter blade to a small jet of air and the aerodynamic pitching moment is measured and the coeflicient determined at this point. The trailing edge tab is then bent at the span which is subjected to the jet to correct the pitching moment to the right value. This pitching moment coelficient may be zero or some fixed value for example a coeificient from to or '-0.01 approximately. The trailing edge tab is, therefore, adjusted wherevernecessary throughout the length of the blade so that the'picthing moment coefficient 1 is the same throughout its length. A single blade having the same coefiicient as the blades already on the head may be mounted upon the rotor head without further whirling or adjustment and will track so that there is no need of replacing all of the blades on the head with a matched set or of individually adjusting the inserted blade so that it tracks with the blades mounted on the rotor head. It is clear that if for any reason it is not desired to have the pitching moment coeflicient of the same value throughout the length of the blade, that adjustment of the trailing edge tab may be made so that the coeificient may be any desired value and may vary as desired throughout the length of the blade. All blades of course must vary in the same way.

It is an object of the invention to provide'a simplified and convenient method of testing and determining the aerodynamic pitching moment and the pitching moment coeflicient of a full-sized aerodynamic surface such as a helicopter blade by testing very short spans thereof.

Another object is to determine the aerodynamic pitching moment or coefiicient and adjusta very short span at a time of a full-sized aerodynamic surface or helicopter blade so that the coefiicient is of a desired value.

A still further object is to provide a method for determining the aerodynamic pitching moment or coefiicient of a very short span of a full-sized aerodynamic surface of considerably greater length than the short span and thereafter to adjust the same to a desired coeliicient and to repeat the process throughout the span of the surface or blade.

Another object is to construct a simple and inexpensive apparatus for practicing the simplified methods of testing and adjusting the aerodynamic pitching moment or coefiicient of a short span of a full-sized aerodynamic surface.

Other objects of the invention will be more apparent from the following description when taken in connection with the accompanying drawings illustrating a form of apparatus which may be conveniently used for practicing the method in which:

Figure 1 is a front elevation of an apparatus for determining the aerodynamic pitching moment or coefiicient of an aerodynamic surface such as a helicopter blade so that it may be adjusted to the proper value;

Figure 2 is an enlarged partial sectional view taken on line 22 of Figure 1;

Figure 3 is an enlarged sectional view of securing means for the end of the spar; and

Figure 4 is an end view of a portion of helicopter blade showing an adjustable trailing edge tab.

The apparatus shows the aerodynamic member as a helicopter rotor blade 10 with its spar or shaft 11 projecting from the end thereof the axis of which is located spaced from or at the mean aerodynamic center or chord of the member as desired. The blade spar is mounted on a rotor head for cyclic pitch change of the blade along the axis of the spar. The trailing edge of the blade is adjustable or has an adjusting tab or tabs 12 of any suitable kind and bendable tabs such as of thin metal are known. The tab or tabs usually project beyond the trailing edge of the blade proper and may be adjusted either upwardly or downwardly to change the aerodynamic pitching moment, and hence the moment coeflicient of the blade so that it will be of the proper value.

The blade is mounted in any desired manner so that it is free to move in at least one direction, that is turn angularly on the axis of the spar 11. Preferably the mounting is such that the aerodynamic member or blade has two directions of movement, an angular movement and a lateral pendular movement. Suitable balancing and indicating means are also provided to balance the turning torque or effort created by the air jet on the blade and to indicate the torque moment.

Figure 1 illustrates an end or vertical suspension mounting in which the blade is vertically suspended from the spar 11 such as by a wire 14. A suitable bushing 15 is secured to the end of the spar and the wire is secured thereto such as by a set screw 16. The upper end of the wire is preferably anchored to a support and. is long enough to provide sufficient torsional resiliency to allow the blade to turn easily upon its axis under the influence of the air jet. A wire of about A of an inch in diameter and eighteen inches long is strong enough to support the blade and give suflicient torsional elasticity or sensitivity. A longer wire gives greater torsional elasticity if that should be desirable or a thicker and longer wire gives equal torsional resiliency. The wire suspension gives the blade freedom of movement in two directions. Normally the blade hangs down being acted upon by gravity. The blade however may swing pendularly laterally which is one direction of freedom of movement and it is also free to turn on its axis, or the axis of the spar 11 as provided by the suspending wire 14. The vertical suspension of the blade particularly shown is illustrative and other suspensions are apparent which will accomplish the same purpose.

The support may be an angle plate 17 secured to a post, wall or the like and carries a bushing 18 to receive the wire 14. A set screw 19 clamps the Wire in the bushing. Means are provided to change the angular position of the wire and hence the angular position of the blade known as feathering the aerodynamic member. The supporting bushing may be adjusted by suitable means such as the screws 21 engaging a lever or projection 22 extending from the supporting bushing.

The balancing means to measure or indicate the turning effort or force applied to the blade by the air jet may be of any suitable construction. In the construction shown the wire 14 serves a double function namely as a suspension or supporting means for the blade and also as the means opposing the turning effort created by the jet directed at the blade. The wire is torsionally resilient or is a spring means for resiliently or yieldingly opposing the turning of the blade. The wire supporting means is desirable not only because it serves a double function but also because the frictional forces are zero or nil so that a sensitive mounting is secured. To indicate or measure the pitching force or moment, a pointer 30 is clamped to the spar 11 which pointer passes over a graduated dial 31 carried by a bracket 32 secured to a post, wall etc. and indicates the amount of torque or turning force being applied to the blade to turn the same on its axis. In order to assure centering of the wire the bracket may carry a guiding bushing 33 which is adjacent to the end of the spar and has a hole therethrough to freely pass the wire 14.

Means are provided to project a relatively small jet of air at the leading edge of the blade over a relatively short span of the aerodynamic surface or member which is fullsized as to chord and length. The jet means is moved along the blade or the blade is moved relatively to the jet means in order to subject the entire length of the blade to the jet. The means shown includes a jet nozzle 23 connected with a fan 24 driven by a suitable motor (not shown). Preferably the jet, fan and motor are portable and mounted for movement lengthwise before the leading edge of the blade in any manner such as by mounting the jet mechanism on a sliding or wheeled carriage which rides on tracks 26. Since the blade is hung vertically the jet is moved vertically on the tracks carried by a wall or forming a vertical tower. The portable or movable jet mechanism may be moved along the tracks in any way. It is clear that the blade supporting structure 17 and indicator 31, 32 may be mounted on the portable carriage 25 for longitudinal relative movement between the blade and the jet mechanism.

The dimensions of the jet may vary considerably but a convenient jet nozzle size would be six inches wide by eighteen inches long. This jet is projected toward the blade so that the six inch narrow dimension is spanwise of the blade and the longer dimension extends laterally of the blade. A jet of air of this size projected at a speed of one hundred miles an hour at the leading edge would require about a ten horsepower motor. A larger jet would require a larger fan and motor to produce a suflicient volume of air at the specified air speed so that the size of the jet determines largely how bulky the jet apparatus will be and its cost. Similarly there may be a very wide range of jet velocity which may be used. The air jet speed or velocity may be very low but should be great enough to make readings accurate taking into consideration friction at the support bearing for the blade. Except for bulkiness and cost of jet apparatus, the upper limit of the jet air speed may be the sonic barrier and the lower limit may be about a few miles per hour. Knowing the speed of the air jet and the speed of the blade or aerodynamic member the aerodynamic moment at blade speed can be calculated. Since helicopters today fly at a maximum forward speed of about one-huntired and twenty miles per hour, the air speed of the blade is the rotational speed plus or minus the forward flight speed depending upon whether the blade is advancing or retreating from the direction of flight.

In use the motor for the fan is connected with a source of current and when the air jet has reached full speed, it is moved until the jet is opposite the leading edge of the blade and its stream of air is directed thereon. The blade may create a lift force as a consequence of the air stream being directed thereon because of some inaccuracy in the blade contour. The blade will move laterally pendularly in the air stream until the weight of the blade under the influence of gravity equalizes the lift force. Preferably, the blade is turned or feathered angularly by turning the bushing 18 with the screws 21 to a position with respect to the air jet so that the blade has zero lift and therefore it hangs vertically downward. A scale 29 below the free or lower end of the blade indicates whether or not there has been any pendular movement. It is clear that it is the relative relation between the direction of the air stream and the angle of the blade thereto which determines whether or not there will be a lift force and either one could be changed to make the feathering correction; however, turning of the blade is the simpler operation. To turn the air jet or change its angle of presentation to the leading edge of the blade, it may be mounted upon a pivot 28 in which case the angular adjustment of the bushing 18 may be dispensed with.

With the blade hanging straight down without lift force and with the air jet directed towards or against the leading edge, the blade will turn on its longitudinal or spar axis if there is a pitching moment or force. The amount of pitching moment can be read from the indicator and the pitching moment coeflicient determined. If it is not of the proper value, the trailing edge tab in this section or short span of the blade is adjusted and a second reading taken. Further adjustment is made until the pitching moment is of the proper value which may be zero or some desired value depending upon the type of rotor head and in the correct or proper direction. The air jet may be removed in order to make the adjustment after which it is returned for further measurement and if necessary adjustment until this section of the blade has the desired aerodynamic pitching moment or moment coefficient. When this section or span of the blade indicates the desired pitching moment or pitching moment coefficient, the air jet is moved to another or the next adjacent section of the blade and the same process continued until the entire blade, or that portion governed by the tab depending on length of tab used, has been adjusted to the proper value of aerodynamic pitching moment or coeflicient throughout its length.

All of the blades to be mounted upon the rotor head are adjusted to the same value of aerodynamic pitching moment coeflicient. A set of blades having proper mass distribution which have been adjusted to this same value may be mounted upon a rotor head at equal angles of incidence and they will satisfactorily track with each was other without further adjustment to the aerodynamic tab. In the adjustment it may be necessary to first bend'the trailing edge tab at certain amount and continue this several times i-n the same or opposite directions before this section or span of the blade is adjusted to the proper value. The trailing edge tab should be of such character to withstand repeated bendings.

The air jet. preferably delivers a fixed speed of airstream mentioned above as being'about one hundred miles an hour. Sincea helicopter blade rotates on a rotor head, the inner end of the blade adjacent the rotating axis has a smaller air speed. The air speed so far .as rotation is concerned increases as the distance increases from the center of rotation outwardly to the outer tip of the blade. Actual operating speed for different sections or short spans therefore differs from the fixed speed of the air jet described as preferred above. It is clear that the speed of the air jet may be low at the inner end and speeded up by speeding up the motor as the jet is moved from the inner end of the blade towards the outer end. It is simpler, however, to calculate the relative speed of the air at various positions of the blade and introduce a correction factor into the aerodynamic moment reading determined from the balance indicator. With the air or jet speed known and since aerodynamic pitching moment is proportional to the square of the velocity, the calculations are readily made. A chart or graph could be prepared which would give the correction factor for each short span of the blade and the trailing edge tab 12 is adjusted so that the pitching momentand the moment coefficient is a proper value taking factor.

The invention is presented to fill a need forimprovements in a method of determining and adjusting the aerodynamicpitching moment of a full-sized aerodynamic surface and machine therefor. It is understood that various modifications in structure, as well as changes in mode of operation, assembly, and manner of use, may and often do occur to those skilled in the art, especially after benefiting from the teachings of an invention. Hence it will be understood that this disclosure is illustrative of preferred means of embodying the invention in useful form by explaining the construction, operation and advantages thereof.

What is claimed is:

l. A method of testing the aerodynamic pitching moment or coeflicient of a full-sized aerodynamic member such as a rotary wing aircraft blade having a pitching moment adjusting element to correct for inaccuracies of airfoil contour comprising supporting the aerodynamic member for movement about the axis of its mean aerodynamic center, creating an air stream having a width of very short span relatively to the length of the aerodynamic member, directing the air stream at the leading edge of the aerodynamic member and successively at a plurality of positions along the length thereof, angularly adjusting one of the elements including the aerodynamic member and air stream to eliminate lift effects of the air foil if such should appear, opposing yieldingly any angular displacement of the aerodynamic member, and measuring the angular displacement of the aerodynamic member at each position.

2. A method as in claim 1 including adjusting a blade pitching moment adjusting element to change the aerodynamic pitching moment to a desired value.

3. A method as in claim 1 in which the angular adjustment is made to the aerodynamic member.

4. A method as in claim 1 in which the aerodynamic adjusting element is a trailing edge tab including adjusting the trailing edge tab of the aerodynamic member within the area subjected to the air stream to change the pitching moment.

5. A method of testing the aerodynamic pitching moment or coefficient of a full-sized aerodynamic meminto consideration the correction her such as a rotary wing aircraft blade having a pitching moment adjusting element to correct for inaccuracies of airfoil contour comprising supporting the aerodynamic member for movement about the axis of its mean aerodynamic center and pendularly laterally, creating an air stream having a width of very short span relatively to the length of the aerodynamic member, directing the air stream at the leading edge of the areodynamic member and successively at a plurality of positions along the length thereof, angularly adjusting one of the elements including the aerodynamic member and air stream to eliminate lift effects of the air foil if such should appear, opposing yieldingly any angular displacement of the aerodynamic member, and measuring the angular displacement of the aerodynamic member at each position.

I 6. A method as in claim 5 including adjusting an aerodynamic pitching moment adjusting element to change the pitching moment to the desired value.

7. A method of testing the aerodynamic pitching moment or coefficient of a full-sized aerodynamic member such as a rotary wing aircraft blade having a pitching moment adjusting element to correct for inaccuracies of airfoil contour comprising supporting the blade for movement in two directions namely about the axis of its mean aerodynamic center and pendularly laterally, creating an air stream having a Width of very short span relatively to the blade length, directing the air stream at the leading edge of the blade and successively at a plurality of positions along the length thereof, angularly adjusting the blade to eliminate lift effects of the air foil if such should appear, opposing yieldingly any angular displacement of the aerodynamic member, and measuring the angular displacement of the aerodynamic member at each position.

8. A method as in claim 7 including adjusting a blade element to change the aerodynamic pitching moment to a desired value including zero.

9. A method as in claim 7 in which the aerodynamic adjusting element is a trailing edge tab including adjusting a trailing edge tab of the blade Within the area subjected to the air stream to change the pitching moment.

10. A method of adjustment of the aerodynamic pitching moment coefficient of a full-sized aerodynamic member such as a rotary wing aircraft blade having a trailing edge tab for adjusting the pitching moment to correct for inaccuracies of airfoil contour comprising supporting the blade for free movement about the axis of its mean aerodynamic center, creating an air stream having a width of short span relatively to the blade length, directing the air stream at the leading edge of the blade, angularly adjusting one of the elements including the aerodynamic member and air stream to eliminate lift effects of the air foil if such should appear, opposing yieldingly angular displacement of the aerodynamic member, indicating the angular displacement of the aerodynamic member, adjusting the trailing edge tab of the blade where subjected to the air stream, and repeating the steps throughout the length of the aerodynamic member.

11. Apparatus for determining the areodynamic pitching moment of a full sized aerodynamic member such as a rotary wing aircraft blade having a pitching moment adjusting element to correct for inaccuracies of airfoil contour comprising means supporting the aerodynamic member for free angular movement about the axis of its mean aerodynamic center and for free lateral movement, means providing an air jet having a Width of very short span relatively to the length of the aerodynamic member and located with respect to the supporting means so as to direct the air stream at the leading edge of the aerodynamic member, means to change the angular relation of one of the elements including the supporting means and the air jet to eliminate lift effects of the airfoil if such should appear, means mounting one of the elements including the air jet means and the supporting means for movement with respect to the other whereby the jet is applied along the leading edge of the aerodynamic member, and means connected with the aerodynamic member for indicating the angular movement thereof.

12. Apparatus as in claim 11 including means mounting the air jet means for movement relatively to the supporting means.

13. Apparatus as in claim 11 in which the means supporting the aerodynamic member supports the same freely for pendular movement.

14. Apparatus for determining the aerodynamic pitching moment or coefiicient of a full sized aerodynamic member such as a rotory wing aircraft blade having a pitching moment adjusting element to correct for inaccuracies of airfoil contour comprising means supporting the aerodynamic member for free angular movement about the axis of its mean aerodynamic center, an air jet means having a width of very short span relatively to the length of the aerodynamic member and located with respect to the supporting means to direct the air stream at the leading edge of an aerodynamic member, means to change the angular relation of one of the elements including the supporting means and the air jet means to eliminate lift effects of the air foil if such should appear, means mounting the air jet means for movement with respect to the supporting means to move the jet means along the leading edge of the aerodynamic member, and means connected with the aerodynamic member for resisting angular movement and for indicating the amount of angular movement.

15. Apparatus as in claim 14 in which the means supporting the aerodynamic member supports the same with freedom for pendular movement.

ill

16. Apparatus for determining the aerodynamic pitching moment coefficient of a full-sized aerodynamic member such as a rotary wing aircraft blade having a spar and a pitching moment adjusting element to correct for inaccuracies of airfoil contour comprising means support ing the aerodynamic member for free angular movement about the axis of its spar and for pendular movement including means to adjust the angular position of the supporting means to eliminate lift effects of the airfoil if such should appear, an air jet means having a width of very short span relatively to the length of the aerodynamic member and located with respect to the supporting means to direct the air stream at the leading edge of the aerodynamic member, means mounting the air jet means for movement with respect to the sup porting means to move the jet means along the leading edge of the aerodynamic member, and means connected with the aerodynamic member for indicating the angular movement.

17. Apparatus as in claim 16 in which the supporting means includes a torsionally resilient wire.

18. Apparatus as in claim 17 in which the means for indicating the angular movement includes a pointer to be attached to the aerodynamic member and graduations adjacent to the pointer.

References Cited in the file of this patent UNITED STATES PATENTS 2,039,211 Caldwell Apr. 28, 1936 2,552,739 Roberts May 15, 1951 FOREIGN PATENTS 954,640 France June 13, 1949 

